The Latin word basilica (derived from Greekβασιλικὴ στοά, lit. "royal stoa", serving as the tribunal chamber of a king) has three distinct applications in modern English. The word was originally used to describe an ancient Roman public building where courts were held, as well as serving other official and public functions. To a large extent these were the town halls of ancient Roman life. The basilica was centrally located in every Roman town, usually adjacent to the main forum. These buildings, an example of which is the Basilica Ulpia, were rectangular, and often had a central nave and aisles, usually with a slightly raised platform and an apse at each of the two ends, adorned with a statue perhaps of the emperor, while the entrances were from the long sides.[1][2]

By extension the name was applied to Christian churches which adopted the same basic plan and it continues to be used as an architectural term to describe such buildings, which form the majority of church buildings in Western Christianity, though the basilican building plan became less dominant in new buildings from the later 20th century. Later, the term came to refer specifically to a large and important Roman Catholic church that has been given special ceremonial rights by the Pope.

Remains of the Basilica of Maxentius and Constantine in Rome. The building's northern aisle is all that remains.

Floor plan of the Basilica of Maxentius and Constantine

The Roman basilica was a large public building where business or legal matters could be transacted. The first basilicas had no religious function at all. As early as the time of Augustus, a public basilica for transacting business had been part of any settlement that considered itself a city, used in the same way as the late medieval covered market houses of northern Europe, where the meeting room, for lack of urban space, was set above the arcades, however. Although their form was variable, basilicas often contained interior colonnades that divided the space, giving aisles or arcaded spaces on one or both sides, with an apse at one end (or less often at each end), where the magistrates sat, often on a slightly raised dais. The central aisle tended to be wide and was higher than the flanking aisles, so that light could penetrate through the clerestory windows.

The oldest known basilica, the Basilica Porcia, was built in Rome in 184 BC by Cato the Elder during the time he was Censor. Other early examples include the basilica at Pompeii (late 2nd century BC).

Probably the most splendid Roman basilica (see below) is the one begun for traditional purposes during the reign of the pagan emperor Maxentius and finished by Constantine I after 313 AD.

In the Roman Imperial period (after about 27 BCE), a basilica for large audiences also became a feature in palaces. In the 3rd century AD, the governing elite appeared less frequently in the forums.

They now tended to dominate their cities from opulent palaces and country villas, set a little apart from traditional centers of public life. Rather than retreats from public life, however, these residences were the forum made private.(Peter Brown, in Paul Veyne, 1987)

Seated in the tribune of his basilica, the great man would meet his dependent clientes early every morning.

A private basilica excavated at Bulla Regia (Tunisia), in the "House of the Hunt", dates from the first half of the 5th century. Its reception or audience hall is a long rectangular nave-like space, flanked by dependent rooms that mostly also open into one another, ending in a semi-circular apse, with matching transept spaces. Clustered columns emphasised the "crossing" of the two axes.

In the 4th century, once the Imperial authorities had decriminalised Christianity with the 313 Edict of Milan, and with the activities of Constantine the Great and his mother Helena, Christians were prepared to build larger and more handsome edifices for worship than the furtive meeting-places (such as the Cenacle, cave-churches, house churches such as that of the Roman consuls John and Paul) they had been using. Architectural formulas for temples were unsuitable, for their pagan associations, and because pagan cult ceremonies and sacrifices occurred outdoors under the open sky in the sight of the gods, with the temple, housing the cult figures and the treasury, as a backdrop. The usable model at hand, when Constantine wanted to memorialise his imperial piety, was the familiar conventional architecture of the basilicas.[6]

Floor plan of a Christian church of basilical form, with the transept shaded. Either the western part of the nave or the choir may have a hall structure instead. The choir also may be aisleless

There were several variations of the basic plan of the secular basilica, always some kind of rectangular hall, but the one usually followed for churches had a central nave with one aisle at each side and an apse at one end opposite to the main door at the other end. In, and often also in front of, the apse was a raised platform, where the altar was placed, and from where the clergy officiated. In secular building this plan was more typically used for the smaller audience halls of the emperors, governors, and the very rich than for the great public basilicas functioning as law courts and other public purposes.[7] Constantine built a basilica of this type in his palace complex at Trier, later very easily adopted for use as a church. It is a long rectangle two storeys high, with ranks of arch-headed windows one above the other, without aisles (there was no mercantile exchange in this imperial basilica) and, at the far end beyond a huge arch, the apse in which Constantine held state.

Putting an altar instead of the throne, as was done at Trier, made a church. Basilicas of this type were built in western Europe, Greece, Syria, Egypt, and Palestine, that is, at any early centre of Christianity. Good early examples of the architectural basilica include the Church of the Nativity at Bethlehem (6th century AD), the church of St Elias at Thessalonica (5th century AD), and the two great basilicas at Ravenna.

Thus, a Christian symbolic theme was applied quite naturally to a form borrowed from civil semi-public precedents. The first great Imperially sponsored Christian basilica is that of St John Lateran, which was given to the Bishop of Rome by Constantine right before or around the Edict of Milan in 313 and was consecrated in the year 324. In the later 4th-century, other Christian basilicas were built in Rome: Santa Sabina, and St Paul's Outside the Walls (4th century), and later St Clement (6th century).

A Christian basilica of the 4th or 5th century stood behind its entirely enclosed forecourt ringed with a colonnade or arcade, like the stoa or peristyle that was its ancestor or like the cloister that was its descendant. This forecourt was entered from outside through a range of buildings along the public street. This was the architectural ground-plan of St Peter's Basilica in Rome, until in the 15th century it was demolished to make way for a modern church built to a new plan.

In most basilicas, the central nave is taller than the aisles, forming a row of windows called a clerestory. Some basilicas in the Caucasus, particularly those of Armenia and Georgia, have a central nave only slightly higher than the two aisles and a single pitched roof covering all three. The result is a much darker interior. This plan is known as the "oriental basilica", or "pseudobasilica" in central Europe.

Gradually, in the early Middle Ages there emerged the massive Romanesque churches, which still kept the fundamental plan of the basilica.

In the Eastern Orthodox Church, in general, the basilica is a mere architectural description of churches built in the ancient style. It bears no significance with regard to precedence or importance of the particular building or clerics associated with it. Eastern basilicas may be single-naved, or have the nave flanked by one or two pairs of lower aisles; it may have a dome in the middle: in this case it is called a "domed basilica".

In Romania, the word for church both as a building and as an institution is biserică, derived from the term basilica.

The Early Christian purpose-built basilica was the cathedral basilica of the bishop, on the model of the semi-public secular basilicas, and its growth in size and importance signalled the gradual transfer of civic power into episcopal hands, which was under way in the 5th century. Basilicas in this sense are divided into classes, the major ("greater") basilicas and the minor basilicas; there are three other papal and several pontifical minor basilicas in Italy, and over 1,400 lesser basilicas around the world.

Churches designated as papal basilicas, in particular, possess a papal throne and a papal high altar, at which no one may celebrate Mass without the pope's permission.[8]

Numerous basilicas are notable shrines, often even receiving significant pilgrimages, especially among the many that were built above a confessio or the burial place of a martyr – although this term now usually designates a space before the high altar that is sunk lower than the main floor level (as in the case in St Peter's and St John Lateran in Rome) and that offer more immediate access to the burial places below.

The papal or major basilicas outrank in precedence all other churches. Other rankings put the cathedral (or co-cathedral) of a bishop ahead of all other churches in the same diocese, even if they have the title of minor basilica. If the cathedral is that of a suffragan diocese, it yields precedence to the cathedral of the metropolitan see. The cathedral of a primate is considered to rank higher than that of other metropolitan(s) in his circonscription (usually a present or historical state). Other classifications of churches include collegiate churches, which may or may not also be minor basilicas.

To this class belong only the four great papal churches of Rome, which among other distinctions have a special "holy door" and to which a visit is always prescribed as one of the conditions for gaining the Roman Jubilee. Upon relinquishing in 2006 the title of Patriarch of the West, Pope Benedict XVI renamed these basilicas from "Patriarchal Basilicas" to "Papal Basilicas".

St. John Lateran, also called the Lateran Basilica, is the cathedral of the Bishop of Rome, the Pope.

St. Peter's, also called the Vatican Basilica, is a major pilgrimage site, built over the burial place of Saint Peter.

The four papal or major basilicas were formerly known as "patriarchal basilicas". Together with the minor basilica of St Lawrence outside the Walls, they were associated with the five ancient patriarchal sees of Christendom (see Pentarchy): St John Lateran was associated with Rome, St Peter's with Constantinople (present-day Istanbul), St Paul's with Alexandria (in Egypt), St Mary Major with Antioch (the Levant) and St Lawrence with Jerusalem.

The privileges attached to the status of minor basilica, which is conferred by papal brief, include a certain precedence before other churches, the right of the conopaeum (a baldachin resembling an umbrella; also called umbraculum, ombrellino, papilio, sinicchio, etc.) and the bell (tintinnabulum), which are carried side by side in procession at the head of the clergy on state occasions, and the cappa magna which is worn by the canons or secular members of the collegiate chapter when assisting at the Divine Office.[8] In the case of major basilicas these umbraculae are made of cloth of gold and red velvet, while those of minor basilicas are made of yellow and red silk—the colours traditionally associated with both the Papal See and the city of Rome.

Tintinnabulum and conopaeum, one of the privileges granted to a basilica

Every year, on 13 May and 13 October, the significant dates of the Fatima apparitions, pilgrims fill the country road that leads to the Sanctuary of Our Lady of Fátima with crowds that approach one million on each day.[10] In December 2009 the Basilica of Our Lady of Guadalupe set a new record with 6.1 million pilgrims during Friday and Saturday for the anniversary of Our Lady of Guadalupe.[5]